1. Field of the Invention
The present invention relates to a laser beam welding apparatus for welding a workpiece at a plurality of locations thereon efficiently with a laser beam.
1. Description of the Related Art
It has heretofore been customary to rely on resistance spot welding robots for welding inner and outer panels of automobile bodies. The resistance spot welding robots have a welding gun movable for successively welding various locations on an automobile body which has been loaded into a welding station.
Since the welding gun is relatively heavy, however, it takes a relatively long time to move the welding gun from one welding location to another on an automobile body, and therefore the welding gun cannot weld the automobile body efficiently. One solution to this problem is to use a plurality of resistance spot welding robots for welding an automobile body within a reduced period of time.
Using such plural resistance spot welding robots poses another drawback in that it results in increased investments in the welding facility and also in a welding line of increased length because of a large installation space required by the resistance spot welding robots. Because the welding gun of resistance spot welding robots has welding tips that are held in contact with a workpiece to be welded, it is necessary to take into account various adverse effects which the welding tips suffer, e.g., adhesion to the workpiece and wear of the welding tips. Specifically, the welding tips need to be monitored at all times, and when the welding tips adhere to the workpiece, the welding line have to be shut off, and suitable actions have to be taken to remove the welding tips from the workpiece. The welding tips should also be dressed periodically to keep themselves under conditions for optimum welding performance.
To eliminate the foregoing difficulties imposed by the resistance spot welding robots, it is been proposed to employ a welding robot for welding automobile bodies with a laser beam as disclosed in Japanese laid-open patent publication No. 4-220187. According to the disclosed welding robot, a laser beam application head mounted on the tip end of an arm of the welding robot is moved closely to a location to be welded on the automobile body and then welds the location on the automobile body. Inasmuch as the welding robot uses a laser beam, it is free of the problems of adhesion to the workpiece and wear of welding tips.
However, the laser beam application head cannot be moved at a high speed because it needs to be positioned in the vicinity of a desired location to be welded in a manner to coordinate with movement of the welding robot.
Japanese patent publication No. 4-36792 reveals a plurality of welding heads that can be positioned near a location to be welded on a workpiece. The angle of a plane mirror is adjusted to select a desired one of the welding heads, and the selected welding head is used to apply a laser beam to the location to be welded on the workpiece for thereby welding the workpiece at the location. The disclosed welding apparatus can weld the workpiece at a plurality of locations in a relatively short period of time.
One problem with the disclosed welding apparatus is that since the welding heads are fixed with respect to the workpiece, they cannot perform a seam welding process, which is a continuous welding process, though they can spot-weld the workpiece. Furthermore, because as many welding heads are required as the number of locations to be welded, the welding apparatus is expensive to manufacture. The positions and number of welding heads need to be changed depending on the type of workpieces to be welded, and such a changing process is tedious and time-consuming to carry out. In addition, the welding heads that are positioned closely to locations to be welded on a workpiece tend to be smeared by spatters produced when the welding heads weld the workpiece, resulting in a reduction in the accuracy with which the welding heads weld the workpiece.